Preparation of Alkynes: Elimination Reactions of Dihalides

Alkynes can be prepared by the elimination of HX from alkyl halides in a similar manner as alkenes.  Treatment of a 1,2-dihaloalkane (a vicinal dihalide) with an excess amount of a strong base such as KOH or NaNH₂ results in a twofold elimination of HX and formation of an alkyne . The starting vicinal dihalides are themselves readily available by addition of Br₂ or Cl₂ to alkenes. Thus, the overall halogenation/dehydrohalogenation sequence makes it possible to go from an alkene to an alkyne. For example, diphenylethylene is converted into diphenylacetylene by reaction with Br₂ and subsequent base treatment.

The twofold dehydrohalogenation takes place through a vinylic halide intermediate, which suggests that vinylic halides themselves should give alkynes when treated with strong base. (Remember: A vinylic substituent is one that is attached to a double-bond carbon.) This is indeed the case. For example: